Add missing header status code (linked to backend issue #19)

helpers/LazyVal.ts

 class LazyVal<T> {
     private val: T | undefined = undefined;
-    private valLoader: () => Promise<T> | T;
+    private readonly valLoader: () => Promise<T> | T;
 
     constructor(valLoader: () => Promise<T> | T) {
         this.valLoader = valLoader;